Transconductance Amplifier using S.M.P.S

My design is greatly inspired by Menno Van Der Veen ideas published in his Trans Tube

Amplifier book.

2 years ago, I’ve already assembled a mono prototype using 6v6 tubes in a Push-Pull

configuration, the output transformer was a low cost R-TU101 (E.I type) , a “modern”

transformer re-issued by the French RADIOFIL magazine from an old AUDAX TU101 ®.

The power supplies used were linear ones. The result was already astonishingly good to my

hears taking into account the simplicity of the schematic!

That time, for compactness reasons, I started to assemble a prototype using EL84 tubes, the

same output transformer and only a set of switching mode power supplies. I found a small

DC/DC dedicated to the plates voltage, delivering from 15V D.C around 300V , of course due

to its small size, it could only deliver 50mA, just enough to my needs cause I only want

around 1W of audio power using loudspeakers of SPL 92dB@1W in a 11m² room.

Amplifier schematic:

The principle is rather close to Menno’s Transie 2 amplifier, concerning the current sources I

used FETs DN2530 ® from Supertex Inc. As a phase splitter I use a dual DIFET OPA2107 ®

from Burr Brown .See all datasheets in annex. EL84 tubes bias is automatic and set around

22mA for a plate voltage around 250V. On the prototypes, I added 2 pots in serial with the

feedback resistors to adjust the differential voltage between the tubes grids; they could be

replaced further on with 1% resistors. Nevertheless as noticed by Menno in his book, DC

coupling is sensitive to AC line fluctuation, despite the use of SMPS, I could observe this to.

Power supplies schematic:

To obtain around 250-300V , I needed to use 2 DC/DC RECOM modules. A temporized relay

is used and set to 45 s before applying the H.V.

As expressed above, I was firstly looking for a compact design because I wanted to reuse this

Chinese hybrid amplifier rack:

And secondly I was interested in the use of SMPS in my DIY project.

Measurements:

First square wave shapes at the output load:

Magenta trace is the input voltage, Yellow one the differential Grids voltage and the blue

one the output voltage on the 5 Ohms, obviously it rings a lot!

I had to add 2x10pF in // with the feedback resistors to damp this ringing:

Of course with these 10pF capacitors, the sine response bandwidth falls @-3dB from 90KHz

to 50KHz @1W.

Output signal rise time:

Let’s have a look to the spectrum of the output signal @1W: for a 1KHz sine input:

With a wider span, we don’t observe within a 70dB dynamic range any spurious peaks

coming from the various SMPS:

How does it sound? Well, one more time the feeling was very good, as I wrote to Menno,

from my present EL84 Push-Pull amplifier, I never felt so real and clear neither Stevie Ray

Vaughan nor Buddy Guy Mi guitar string like I did with this transimpedance amplifier, bass

are also tighten!

eric.rabeuf@wanadoo.fr

Annex: